Structural, vibrational, and electronic topological transitions of Bi1.5Sb0.5Te1.8Se1.2 under pressure

Topological insulators have been the subject of intense research interest due to their unique surface states that are topologically protected against scattering or defects. However, the relationship between the crystal structure and topological insulator state remains to be clarified. Here, we show the effects of hydrostatic pressure on the structural, vibrational, and topological properties of the topological insulator Bi1.5Sb0.5Te1.8Se1.2 up to 45 GPa using X-ray diffraction and Raman spectroscopy in a diamond anvil cell, together with first-principles theoretical calculations. Two pressure-induced structural phase transitions were observed: from ambient rhombohedral R (3) over barm phase to a monoclinic C2/m phase at similar to 13 GPa, and to a disordered I4/mmm phase at similar to 22 GPa. In addition, the alloy undergoes several electronic transitions within the R (3) over barm phase: indirect to direct bulk band gap transition at similar to 5.8 GPa, bulk gap closing with an appearance of Dirac semimetal (DSM) state at similar to 8.2 GPa, and to a trivial semimetal state at similar to 12.1GPa. Anomalies in c/a ratio and Raman full width at half maximum that coincide with the DSM phase suggest the contribution of electron-phonon coupling to the transition. Compared to binary end members Bi2Te3, Bi2Se3, and Sb2Te3, the structural phase transition and anomaly were observed at higher pressures in Bi1.5Sb0.5Te1.8Se1.2. These results suggest that the topological transitions are precursors to the structural phase transitions. Published by AIP Publishing.